How can I add mass to part of a cloth simulation?
Ive spoken briefly with some devs who suggested this feature could be added to the cloth sim again in future. In the meantime, I did manage to create a cheat that works well enough in this case:

Added additional geometry at or near the bottom of the hem. In this case more or less a dense bunch of 10 loops within a couple of centimetres;
Assign these additional verts to a vertex group, and excluded them from self-collision, and;
assigned them a new transparent shader so they don't show in renders.

This made a distinct difference and got basically the result I wanted. Still hoping for a more elegant solution via the devs though, as there are multiple cases where vertex weighting mass would have application.
Hard to post animated visual results yet, but feel free to keep an eye on @LMODanimation or thelovemeordie.com if you want to see more soon.
Thanks for looking!